1. Field of the Invention
This invention is directed to frequency converters which convert multiphase electrical power of one frequency to multiphase power of another frequency. More particularly, it is directed to such frequency converters for delivering multiphase power to unbalanced multiphase loads.
2. Background Information
There are many applications where it is desirable to have multiphase electric power at a controlled frequency. These applications include generating multiphase power of constant frequency from a variable frequency source, generating variable frequency power from a constant frequency source, and applications where the source and load frequency vary. One application of the first type is an airborne power generating system where electrical power of a fixed frequency is generated from the output of a generator which is driven at a variable speed dependent upon the speed of the aircraft engine.
U.S. Pat. No. 4,439,823 discloses a multiphase unrestricted frequency converter (UFC) fed by a current source such as a synchronous generator driven at a variable frequency to supply power at a constant frequency to a multiphase load having the impedance characteristics of a voltage source. Bidirectional switches in the frequency converter are connected directly between each phase of the current source and corresponding phases of the load. Even though more than one source phase is supplying power to an output phase at the same time, no input or output filters or interphase transformers are required because currents applied to the output lines simultaneously are added together to generate the particular output phase current. This arrangement works fine when the impedances of the plural phases of the load are balanced. As long as the output impedances are balanced, the output voltages are balanced. However, unequal load impedances result in unequal load output voltages. Present state-of-the-art voltage control methods do not apply to the current fed unrestricted frequency converter, because they require voltage source inputs.
There is a need therefore for a current fed unrestricted frequency converter which can supply current from a multiphase current source to a multiphase voltage source type load directly and maintain balanced voltages on the load despite imbalances in load impedance.